Separations of nitrogen from admixture with other gases such as oxygen, hydrogen and argon are important industrial process. In such processes the objective may be either to obtain a product gas enhanced in nitrogen or a product from which nitrogen as an undesired constituent has been removed. One of the more important of these commercial scale processes is the separation of air to obtain nitrogen and oxygen. In 1985, six hundred forty-seven billion cubic feet of nitrogen and 380 billion cubic feet of oxygen were produced in the United States alone.
Most of the nitrogen and oxygen derived from air is produced by cryogenic rectification in which air is cooled to temperatures near the normal boiling point of the components and treated in fractionation columns, usually requiring a large number of liquid-gas contact means such as perforated trays. The significant capital costs of the cryogenic separation systems is justified only where large quantities of the product are required, such as oxygen in the manufacture of steel. For operations with smaller requirements, oxygen and nitrogen can also be produced by pressure swing adsorption (PSA) processes. In PSA processes, compressed air is pumped through a fixed bed of an adsorbent exhibiting an adsorptive preference for one of the main constituents whereby an effluent product stream enhanced in the non-adsorbed (or lesser adsorbed) constituent is obtained. Compared to the cryogenic processes, PSA air separation processes require relatively simple equipment and are relatively easy to maintain. PSA processes, however, have lower product recovery and higher energy consumption than the cryogenic processes. For these reasons, improvements in the adsorption processes remain important goals. One principal means of improvement is the discovery and development of better adsorbents.